The 999 telephone call was made at 0606 BST from a telephone box on Well Hall Road, south-east London - near where Stephen Lawrence was murdered in April 1993.

Deputy Assistant Commissioner Alan Fry: "We will take it with the utmost seriousness"

But Scotland Yard detectives said they were keeping an open mind about the motive behind Saturday afternoon's attack which injured dozens of people and have not ruled out the phone call being a hoax.

The head of the Metropolitan Police's anti-terrorist branch, Deputy Assistant Commissioner Alan Fry, said: "This call should be taken with extreme caution.

"This line of inquiry is being taken very seriously but there is absolutely no evidence and there is no intelligence at this time to support this claim, and I can only reiterate to the public that no motive has been ruled out at this time of the investigation.

"We are keeping a completely open mind and we cannot rule out the fact that this may have been a hoax call."

The door of a delivery van was blown off in the blast

Mr Fry revealed that detectives were studying close circuit-television footage taken both at Well Hall Road, Eltham, and near the bomb site at Brixton, south London.

A number of people were seen moving the black Head sports bag in the moments running up to the explosion.

But he said the studying of the pictures was a long process and refused to be drawn on any possible suspects that had been identified in the video footage.

"If we had certainty you could be sure I would making a more direct appeal to you in regard of description of suspects", he said.

The bomb, which was first placed next to a bus stop at about 100 BST, contained between six and 10lbs of nails.

Peter Gould: "The intention was clearly to kill or maim"

Although Combat 18 is not well known in the Brixton area, Mr Fry said there were few areas in the capital where the group's activities had not been known.

He said all lines of inquiry would be followed, but stressed: "It doesn't mean that the caller was a member of Combat 18 necessarily, it doesn't mean that Combat 18 were responsible for this."

Lee Jasper, Secretary of the National Black Alliance, said: "This is racial terrorism on a scale that we have not seen in this country before.

"Racist and terrorist organisations like Combat 18 must be immediately banned and those found to be in membership of the organisation should be apprehended.

"The organisation exists to carry out acts of racist violence and terror."

The National Assembly Against Racism's Chairman Kumar Murshid said: "Black communities will judge the police on how quickly those responsible are apprehended."

Doubts

The two groups are organising a protest meeting in Brixton on 27 April, and a march from Brixton to Downing Street on 1 May.

Steve Silver, co-editor of the anti-facist magazine Searchlight, said the bombing would mark a new set of tactics by Combat 18, but expressed doubts that it really was the organisation's work.

He said: "The unpleasantness of it all is certainly the sort of thing you would expect from Combat 18, and if they were going to carry out such an attack then Brixton is the sort of place they would do it.

"But they have not carried out this kind of bombing before which makes me feel it is unlikely to be them, unless they are trying something new."

Alex Owolade, chairman of local anti-racism campaginers the Movement for Justice, said: "Such a well-known site could not simply have been a random choice.

"Regardless of whether this outrage was the work of a deranged individual or an organised group, it was clearly and attack on the black community."

Forensic scientists search for clues

Meanwhile, a security guard caught in the attack has described from his hospital bed how shoppers did not believe him when he tried to warn them about the device.

Aiah Menjor, who works at the Iceland store where the nail bomb was left on Saturday, was injured by nails from the blast as he tried to clear people away from the scene.

Mr Menjor said: "Nobody took it seriously because people were saying to me 'oh man, look, we have never had this before and we don't expect a bomb to be here. This is not a bomb'.

Aiah Menjor describes the moment the bomb went off

"Even when I said 'move' they wouldn't move. They would say 'no, we don't expect this in Brixton so this is not a bomb'."

Mr Menjor is being treated at St Thomas's hospital, south London.There are fears that he may lose the sight in his left eye.

Of those injured in the blast, 14 are still in hospital, with four giving doctors cause for concern.

Two men, being treated in King's College Hospital, may lose their sight.

A 16-year-old girl is being treated in St Thomas's hospital, where she was operated on for shrapnel injuries on Sunday.

A 23-month-old boy in the same hospital is in a stable condition after an operation to remove a four-inch nail from his skull.